本系列的上一篇介绍了 Sorted Table 的构建和读取过程。当 Sorted Table 不断构建出来时，需要使用适当的方式来组织、管理生成的 .ldb 文件。并且 LevelDB 支持快照，这需要 LevelDB 具有版本管理能力。本篇将分析 LevelDB 版本管理相关的代码。
LevelDB 中的版本管理是针对 Sorted Table 文件的变化的。当有新的内存数据库转为 Sorted Table，或者发生 Compaction 导致有 Sorted Table 增删，都会触发版本的更新。版本管理可以类比 git：
首先来看没有未知依赖的 db/version_edit.h：
struct FileMetaData {
FileMetaData() : refs(0), allowed_seeks(1 << 30), file_size(0) {}
int refs;
int allowed_seeks; // Seeks allowed until compaction
uint64_t number;
uint64_t file_size; // File size in bytes
InternalKey smallest; // Smallest internal key served by table
InternalKey largest; // Largest internal key served by table
};
FileMetaData 记录了 .ldb 文件的元信息，包括允许查找的次数、文件编号 number 和大小 file_size 以及最小和最大的 Key。接下来是 VersionEdit 的定义：
class VersionEdit {
public:
VersionEdit() { Clear(); }
~VersionEdit() = default;
void Clear();
void SetComparatorName(const Slice& name) {
has_comparator_ = true;
comparator_ = name.ToString();
}
void SetLogNumber(uint64_t num) {
has_log_number_ = true;
log_number_ = num;
}
void SetPrevLogNumber(uint64_t num) {
has_prev_log_number_ = true;
prev_log_number_ = num;
}
void SetNextFile(uint64_t num) {
has_next_file_number_ = true;
next_file_number_ = num;
}
void SetLastSequence(SequenceNumber seq) {
has_last_sequence_ = true;
last_sequence_ = seq;
}
void SetCompactPointer(int level, const InternalKey& key) {
compact_pointers_.push_back(std::make_pair(level, key));
}
// Add the specified file at the specified number.
// REQUIRES: This version has not been saved (see VersionSet::SaveTo)
// REQUIRES: "smallest" and "largest" are smallest and largest keys in file
void AddFile(int level, uint64_t file, uint64_t file_size,
const InternalKey& smallest, const InternalKey& largest) {
FileMetaData f;
f.number = file;
f.file_size = file_size;
f.smallest = smallest;
f.largest = largest;
new_files_.push_back(std::make_pair(level, f));
}
// Delete the specified "file" from the specified "level".
void DeleteFile(int level, uint64_t file) {
deleted_files_.insert(std::make_pair(level, file));
}
void EncodeTo(std::string* dst) const;
Status DecodeFrom(const Slice& src);
std::string DebugString() const;
private:
friend class VersionSet;
typedef std::set<std::pair<int, uint64_t>> DeletedFileSet;
std::string comparator_;
uint64_t log_number_;
uint64_t prev_log_number_;
uint64_t next_file_number_;
SequenceNumber last_sequence_;
bool has_comparator_;
bool has_log_number_;
bool has_prev_log_number_;
bool has_next_file_number_;
bool has_last_sequence_;
std::vector<std::pair<int, InternalKey>> compact_pointers_;
DeletedFileSet deleted_files_;
std::vector<std::pair<int, FileMetaData>> new_files_;
};
VersionEdit 包含了几项可编辑属性：
comparator_，比较器的名称；log_number_，日志编号；prev_log_number_，上一个日志编号；next_file_number_，下一个文件编号；last_sequence_，最后的序列号；compact_pointers_，暂时不清楚是什么；delted_files_，删除的文件，记录了 level 和文件号；new_files_，新增的文件，记录了 level 和 FileMetaData。上面几项属性有些还不清楚作用，先搁置不用慌。另外两个重要的接口 EncodeTo 和 DecodeFrom 负责编解码，实现在对应的 .cc 中，不在赘述。接下来，继续看 Version 的定义 db/version_set.h：
class Version {
public:
// Lookup the value for key. If found, store it in *val and
// return OK. Else return a non-OK status. Fills *stats.
// REQUIRES: lock is not held
struct GetStats {
FileMetaData* seek_file;
int seek_file_level;
};
// Append to *iters a sequence of iterators that will
// yield the contents of this Version when merged together.
// REQUIRES: This version has been saved (see VersionSet::SaveTo)
void AddIterators(const ReadOptions&, std::vector<Iterator*>* iters);
Status Get(const ReadOptions&, const LookupKey& key, std::string* val,
GetStats* stats);
// Adds "stats" into the current state. Returns true if a new
// compaction may need to be triggered, false otherwise.
// REQUIRES: lock is held
bool UpdateStats(const GetStats& stats);
// Record a sample of bytes read at the specified internal key.
// Samples are taken approximately once every config::kReadBytesPeriod
// bytes. Returns true if a new compaction may need to be triggered.
// REQUIRES: lock is held
bool RecordReadSample(Slice key);
// Reference count management (so Versions do not disappear out from
// under live iterators)
void Ref();
void Unref();
void GetOverlappingInputs(
int level,
const InternalKey* begin, // nullptr means before all keys
const InternalKey* end, // nullptr means after all keys
std::vector<FileMetaData*>* inputs);
// Returns true iff some file in the specified level overlaps
// some part of [*smallest_user_key,*largest_user_key].
// smallest_user_key==nullptr represents a key smaller than all the DB's keys.
// largest_user_key==nullptr represents a key largest than all the DB's keys.
bool OverlapInLevel(int level, const Slice* smallest_user_key,
const Slice* largest_user_key);
// Return the level at which we should place a new memtable compaction
// result that covers the range [smallest_user_key,largest_user_key].
int PickLevelForMemTableOutput(const Slice& smallest_user_key,
const Slice& largest_user_key);
int NumFiles(int level) const { return files_[level].size(); }
// Return a human readable string that describes this version's contents.
std::string DebugString() const;
private:
friend class Compaction;
friend class VersionSet;
class LevelFileNumIterator;
explicit Version(VersionSet* vset)
: vset_(vset),
next_(this),
prev_(this),
refs_(0),
file_to_compact_(nullptr),
file_to_compact_level_(-1),
compaction_score_(-1),
compaction_level_(-1) {}
Version(const Version&) = delete;
Version& operator=(const Version&) = delete;
~Version();
Iterator* NewConcatenatingIterator(const ReadOptions&, int level) const;
// Call func(arg, level, f) for every file that overlaps user_key in
// order from newest to oldest. If an invocation of func returns
// false, makes no more calls.
//
// REQUIRES: user portion of internal_key == user_key.
void ForEachOverlapping(Slice user_key, Slice internal_key, void* arg,
bool (*func)(void*, int, FileMetaData*));
VersionSet* vset_; // VersionSet to which this Version belongs
Version* next_; // Next version in linked list
Version* prev_; // Previous version in linked list
int refs_; // Number of live refs to this version
// List of files per level
std::vector<FileMetaData*> files_[config::kNumLevels];
// Next file to compact based on seek stats.
FileMetaData* file_to_compact_;
int file_to_compact_level_;
// Level that should be compacted next and its compaction score.
// Score < 1 means compaction is not strictly needed. These fields
// are initialized by Finalize().
double compaction_score_;
int compaction_level_;
};
首先把属性列出来：
vset_，VersionSet 对象指针，该类下文再介绍，搁置；next_ 和 prev_，成对出现，双向链表无疑；refs_，引用计数；files_，每一个 level 中的文件元信息列表；file_to_compact_ 和 file_to_compact_level_，准备合并的文件及其 level，搁置；compaction_score_ 和 compaction_level_，需要执行合并的 level 及打分，搁置。对属性有一个印象就好。接下来拆开来看这个类成员函数的实现 db/version_set.cc，首先看迭代器：
// Return the smallest index i such that files[i]->largest >= key.
// Return files.size() if there is no such file.
// REQUIRES: "files" contains a sorted list of non-overlapping files.
int FindFile(const InternalKeyComparator& icmp,
const std::vector<FileMetaData*>& files, const Slice& key) {
uint32_t left = 0;
uint32_t right = files.size();
while (left < right) {
uint32_t mid = (left + right) / 2;
const FileMetaData* f = files[mid];
if (icmp.InternalKeyComparator::Compare(f->largest.Encode(), key) < 0) {
// Key at "mid.largest" is < "target". Therefore all
// files at or before "mid" are uninteresting.
left = mid + 1;
} else {
// Key at "mid.largest" is >= "target". Therefore all files
// after "mid" are uninteresting.
right = mid;
}
}
return right;
}
// An internal iterator. For a given version/level pair, yields
// information about the files in the level. For a given entry, key()
// is the largest key that occurs in the file, and value() is an
// 16-byte value containing the file number and file size, both
// encoded using EncodeFixed64.
class Version::LevelFileNumIterator : public Iterator {
public:
LevelFileNumIterator(const InternalKeyComparator& icmp,
const std::vector<FileMetaData*>* flist)
: icmp_(icmp), flist_(flist), index_(flist->size()) { // Marks as invalid
}
bool Valid() const override { return index_ < flist_->size(); }
void Seek(const Slice& target) override {
index_ = FindFile(icmp_, *flist_, target);
}
void SeekToFirst() override { index_ = 0; }
void SeekToLast() override {
index_ = flist_->empty() ? 0 : flist_->size() - 1;
}
void Next() override {
assert(Valid());
index_++;
}
void Prev() override {
assert(Valid());
if (index_ == 0) {
index_ = flist_->size(); // Marks as invalid
} else {
index_--;
}
}
Slice key() const override {
assert(Valid());
return (*flist_)[index_]->largest.Encode();
}
Slice value() const override {
assert(Valid());
EncodeFixed64(value_buf_, (*flist_)[index_]->number);
EncodeFixed64(value_buf_ + 8, (*flist_)[index_]->file_size);
return Slice(value_buf_, sizeof(value_buf_));
}
Status status() const override { return Status::OK(); }
private:
const InternalKeyComparator icmp_;
const std::vector<FileMetaData*>* const flist_;
uint32_t index_;
// Backing store for value(). Holds the file number and size.
mutable char value_buf_[16];
};
static Iterator* GetFileIterator(void* arg, const ReadOptions& options,
const Slice& file_value) {
TableCache* cache = reinterpret_cast<TableCache*>(arg);
if (file_value.size() != 16) {
return NewErrorIterator(
Status::Corruption("FileReader invoked with unexpected value"));
} else {
return cache->NewIterator(options, DecodeFixed64(file_value.data()),
DecodeFixed64(file_value.data() + 8));
}
}
Iterator* Version::NewConcatenatingIterator(const ReadOptions& options,
int level) const {
return NewTwoLevelIterator(
new LevelFileNumIterator(vset_->icmp_, &files_[level]), &GetFileIterator,
vset_->table_cache_, options);
}
void Version::AddIterators(const ReadOptions& options,
std::vector<Iterator*>* iters) {
// Merge all level zero files together since they may overlap
for (size_t i = 0; i < files_[0].size(); i++) {
iters->push_back(vset_->table_cache_->NewIterator(
options, files_[0][i]->number, files_[0][i]->file_size));
}
// For levels > 0, we can use a concatenating iterator that sequentially
// walks through the non-overlapping files in the level, opening them
// lazily.
for (int level = 1; level < config::kNumLevels; level++) {
if (!files_[level].empty()) {
iters->push_back(NewConcatenatingIterator(options, level));
}
}
}
FindFile 函数实现了一个简单的二分查找，可以在文件信息列表里快速找到第一个 largest_key >= key 的文件信息编号。而后这里定义了一个文件信息列表的迭代器 Version::LevelFileNumIterator，其实现的功能是将 largest_key 作为 Key，文件编号和大小作为 Value，遍历和检索文件信息列表。该函数将在 Version::NewConcatenatingIterator 中作为第一级迭代器，对应的第二级便是其 Value 对应的 Sorted Table 文件的迭代器 GetFileIterator。这样就可以根据某个 Level 的文件信息列表，生成该 Level 下所有 Sorted Table 数据的迭代器。这还没结束，Version::AddIterators 会将所有 Level 的迭代器组合成一个列表，用来生成一个 MergingIterator 以遍历所有 Level 的数据（实现位于 DBImpl::NewInternalIterator）。仔细体会这个精妙的设计，然后继续来看 Version::Get 的实现：
// Callback from TableCache::Get()
namespace {
enum SaverState {
kNotFound,
kFound,
kDeleted,
kCorrupt,
};
struct Saver {
SaverState state;
const Comparator* ucmp;
Slice user_key;
std::string* value;
};
} // namespace
static void SaveValue(void* arg, const Slice& ikey, const Slice& v) {
Saver* s = reinterpret_cast<Saver*>(arg);
ParsedInternalKey parsed_key;
if (!ParseInternalKey(ikey, &parsed_key)) {
s->state = kCorrupt;
} else {
if (s->ucmp->Compare(parsed_key.user_key, s->user_key) == 0) {
s->state = (parsed_key.type == kTypeValue) ? kFound : kDeleted;
if (s->state == kFound) {
s->value->assign(v.data(), v.size());
}
}
}
}
static bool NewestFirst(FileMetaData* a, FileMetaData* b) {
return a->number > b->number;
}
void Version::ForEachOverlapping(Slice user_key, Slice internal_key, void* arg,
bool (*func)(void*, int, FileMetaData*)) {
const Comparator* ucmp = vset_->icmp_.user_comparator();
// Search level-0 in order from newest to oldest.
std::vector<FileMetaData*> tmp;
tmp.reserve(files_[0].size());
for (uint32_t i = 0; i < files_[0].size(); i++) {
FileMetaData* f = files_[0][i];
if (ucmp->Compare(user_key, f->smallest.user_key()) >= 0 &&
ucmp->Compare(user_key, f->largest.user_key()) <= 0) {
tmp.push_back(f);
}
}
if (!tmp.empty()) {
std::sort(tmp.begin(), tmp.end(), NewestFirst);
for (uint32_t i = 0; i < tmp.size(); i++) {
if (!(*func)(arg, 0, tmp[i])) {
return;
}
}
}
// Search other levels.
for (int level = 1; level < config::kNumLevels; level++) {
size_t num_files = files_[level].size();
if (num_files == 0) continue;
// Binary search to find earliest index whose largest key >= internal_key.
uint32_t index = FindFile(vset_->icmp_, files_[level], internal_key);
if (index < num_files) {
FileMetaData* f = files_[level][index];
if (ucmp->Compare(user_key, f->smallest.user_key()) < 0) {
// All of "f" is past any data for user_key
} else {
if (!(*func)(arg, level, f)) {
return;
}
}
}
}
}
Status Version::Get(const ReadOptions& options, const LookupKey& k,
std::string* value, GetStats* stats) {
stats->seek_file = nullptr;
stats->seek_file_level = -1;
struct State {
Saver saver;
GetStats* stats;
const ReadOptions* options;
Slice ikey;
FileMetaData* last_file_read;
int last_file_read_level;
VersionSet* vset;
Status s;
bool found;
static bool Match(void* arg, int level, FileMetaData* f) {
State* state = reinterpret_cast<State*>(arg);
if (state->stats->seek_file == nullptr &&
state->last_file_read != nullptr) {
// We have had more than one seek for this read. Charge the 1st file.
state->stats->seek_file = state->last_file_read;
state->stats->seek_file_level = state->last_file_read_level;
}
state->last_file_read = f;
state->last_file_read_level = level;
state->s = state->vset->table_cache_->Get(*state->options, f->number,
f->file_size, state->ikey,
&state->saver, SaveValue);
if (!state->s.ok()) {
state->found = true;
return false;
}
switch (state->saver.state) {
case kNotFound:
return true; // Keep searching in other files
case kFound:
state->found = true;
return false;
case kDeleted:
return false;
case kCorrupt:
state->s =
Status::Corruption("corrupted key for ", state->saver.user_key);
state->found = true;
return false;
}
}
};
State state;
state.found = false;
state.stats = stats;
state.last_file_read = nullptr;
state.last_file_read_level = -1;
state.options = &options;
state.ikey = k.internal_key();
state.vset = vset_;
state.saver.state = kNotFound;
state.saver.ucmp = vset_->icmp_.user_comparator();
state.saver.user_key = k.user_key();
state.saver.value = value;
ForEachOverlapping(state.saver.user_key, state.ikey, &state, &State::Match);
return state.found ? state.s : Status::NotFound(Slice());
}
匿名空间中声明了枚举类 SaverState，为查找操作的四种状态：未找到，找到，删除和中断。Saver 负责记录输入的比较器和 user_key，以及输出的 SaverState 和查找得到的 value。SaveValue 作为查找操作的回调函数，将会在 Seek 操作完成后执行，其参数为 SaverState 及键值对。通过判断 user_key 是否一致，对 SaverState 进行更新。
再来看 Version::Get，其调用的 Version::ForEachOverlapping 会根据 smallest_key 和 largest_key 筛选出要查找的文件，再通过回调函数调用 table_cache_->Get 进行查找，如果找到合法的结果则调用回调函数 SaveValue，如果回调得到的结果是 kFound，就可以提前返回了。Level 0 的文件由于可能存在重叠，所以每个文件都需要进行判断；而 Level 1 及以上的文件可以使用 FindFile 二分查找了。
首先来看 VersionSet::Builder 的实现：
// A helper class so we can efficiently apply a whole sequence
// of edits to a particular state without creating intermediate
// Versions that contain full copies of the intermediate state.
class VersionSet::Builder {
private:
// Helper to sort by v->files_[file_number].smallest
struct BySmallestKey {
const InternalKeyComparator* internal_comparator;
bool operator()(FileMetaData* f1, FileMetaData* f2) const {
int r = internal_comparator->Compare(f1->smallest, f2->smallest);
if (r != 0) {
return (r < 0);
} else {
// Break ties by file number
return (f1->number < f2->number);
}
}
};
typedef std::set<FileMetaData*, BySmallestKey> FileSet;
struct LevelState {
std::set<uint64_t> deleted_files;
FileSet* added_files;
};
VersionSet* vset_;
Version* base_;
LevelState levels_[config::kNumLevels];
public:
// Initialize a builder with the files from *base and other info from *vset
Builder(VersionSet* vset, Version* base) : vset_(vset), base_(base) {
base_->Ref();
BySmallestKey cmp;
cmp.internal_comparator = &vset_->icmp_;
for (int level = 0; level < config::kNumLevels; level++) {
levels_[level].added_files = new FileSet(cmp);
}
}
~Builder() {
for (int level = 0; level < config::kNumLevels; level++) {
const FileSet* added = levels_[level].added_files;
std::vector<FileMetaData*> to_unref;
to_unref.reserve(added->size());
for (FileSet::const_iterator it = added->begin(); it != added->end();
++it) {
to_unref.push_back(*it);
}
delete added;
for (uint32_t i = 0; i < to_unref.size(); i++) {
FileMetaData* f = to_unref[i];
f->refs--;
if (f->refs <= 0) {
delete f;
}
}
}
base_->Unref();
}
// Apply all of the edits in *edit to the current state.
void Apply(VersionEdit* edit) {
// Update compaction pointers
for (size_t i = 0; i < edit->compact_pointers_.size(); i++) {
const int level = edit->compact_pointers_[i].first;
vset_->compact_pointer_[level] =
edit->compact_pointers_[i].second.Encode().ToString();
}
// Delete files
for (const auto& deleted_file_set_kvp : edit->deleted_files_) {
const int level = deleted_file_set_kvp.first;
const uint64_t number = deleted_file_set_kvp.second;
levels_[level].deleted_files.insert(number);
}
// Add new files
for (size_t i = 0; i < edit->new_files_.size(); i++) {
const int level = edit->new_files_[i].first;
FileMetaData* f = new FileMetaData(edit->new_files_[i].second);
f->refs = 1;
// We arrange to automatically compact this file after
// a certain number of seeks. Let's assume:
// (1) One seek costs 10ms
// (2) Writing or reading 1MB costs 10ms (100MB/s)
// (3) A compaction of 1MB does 25MB of IO:
// 1MB read from this level
// 10-12MB read from next level (boundaries may be misaligned)
// 10-12MB written to next level
// This implies that 25 seeks cost the same as the compaction
// of 1MB of data. I.e., one seek costs approximately the
// same as the compaction of 40KB of data. We are a little
// conservative and allow approximately one seek for every 16KB
// of data before triggering a compaction.
f->allowed_seeks = static_cast<int>((f->file_size / 16384U));
if (f->allowed_seeks < 100) f->allowed_seeks = 100;
levels_[level].deleted_files.erase(f->number);
levels_[level].added_files->insert(f);
}
}
// Save the current state in *v.
void SaveTo(Version* v) {
BySmallestKey cmp;
cmp.internal_comparator = &vset_->icmp_;
for (int level = 0; level < config::kNumLevels; level++) {
// Merge the set of added files with the set of pre-existing files.
// Drop any deleted files. Store the result in *v.
const std::vector<FileMetaData*>& base_files = base_->files_[level];
std::vector<FileMetaData*>::const_iterator base_iter = base_files.begin();
std::vector<FileMetaData*>::const_iterator base_end = base_files.end();
const FileSet* added_files = levels_[level].added_files;
v->files_[level].reserve(base_files.size() + added_files->size());
for (const auto& added_file : *added_files) {
// Add all smaller files listed in base_
for (std::vector<FileMetaData*>::const_iterator bpos =
std::upper_bound(base_iter, base_end, added_file, cmp);
base_iter != bpos; ++base_iter) {
MaybeAddFile(v, level, *base_iter);
}
MaybeAddFile(v, level, added_file);
}
// Add remaining base files
for (; base_iter != base_end; ++base_iter) {
MaybeAddFile(v, level, *base_iter);
}
#ifndef NDEBUG
// Make sure there is no overlap in levels > 0
if (level > 0) {
for (uint32_t i = 1; i < v->files_[level].size(); i++) {
const InternalKey& prev_end = v->files_[level][i - 1]->largest;
const InternalKey& this_begin = v->files_[level][i]->smallest;
if (vset_->icmp_.Compare(prev_end, this_begin) >= 0) {
fprintf(stderr, "overlapping ranges in same level %s vs. %s\n",
prev_end.DebugString().c_str(),
this_begin.DebugString().c_str());
abort();
}
}
}
#endif
}
}
void MaybeAddFile(Version* v, int level, FileMetaData* f) {
if (levels_[level].deleted_files.count(f->number) > 0) {
// File is deleted: do nothing
} else {
std::vector<FileMetaData*>* files = &v->files_[level];
if (level > 0 && !files->empty()) {
// Must not overlap
assert(vset_->icmp_.Compare((*files)[files->size() - 1]->largest,
f->smallest) < 0);
}
f->refs++;
files->push_back(f);
}
}
};
VersionSet::Builder 中首先定义了一个比较器 BySmallestKey，其会按照文件信息中的 smallest 对文件信息集合 FileSet 中存储的 FileMetaData 排序；定义的结构体 LevelState 中则包括删除的文件编号列表 deleted_files 和新增的文件集合 added_files，VersionSet::Builder 的成员 levels_ 则储存所有 Level 的 LevelState。Builder 的构造和析构完成必要的内存申请和释放，成员还包括版本集 vset_ 和基础版本 base，核心接口为 Apply 和 SaveTo。Apply 函数中先忽略 compact_pointer_ 相关的操作，剩下的就是将 edit 中的增删文件信息插入到 Builder::levels_ 里；而 SaveTo 则是将基础版本 base 中的文件信息和 edit 中的增删文件信息合并，按顺序插入到新版本 v 里。
接着来看 VersionSet 的定义 db/version_set.h：
class VersionSet {
public:
VersionSet(const std::string& dbname, const Options* options,
TableCache* table_cache, const InternalKeyComparator*);
VersionSet(const VersionSet&) = delete;
VersionSet& operator=(const VersionSet&) = delete;
~VersionSet();
// Apply *edit to the current version to form a new descriptor that
// is both saved to persistent state and installed as the new
// current version. Will release *mu while actually writing to the file.
// REQUIRES: *mu is held on entry.
// REQUIRES: no other thread concurrently calls LogAndApply()
Status LogAndApply(VersionEdit* edit, port::Mutex* mu)
EXCLUSIVE_LOCKS_REQUIRED(mu);
// Recover the last saved descriptor from persistent storage.
Status Recover(bool* save_manifest);
// Return the current version.
Version* current() const { return current_; }
// Return the current manifest file number
uint64_t ManifestFileNumber() const { return manifest_file_number_; }
// Allocate and return a new file number
uint64_t NewFileNumber() { return next_file_number_++; }
// Arrange to reuse "file_number" unless a newer file number has
// already been allocated.
// REQUIRES: "file_number" was returned by a call to NewFileNumber().
void ReuseFileNumber(uint64_t file_number) {
if (next_file_number_ == file_number + 1) {
next_file_number_ = file_number;
}
}
// Return the number of Table files at the specified level.
int NumLevelFiles(int level) const;
// Return the combined file size of all files at the specified level.
int64_t NumLevelBytes(int level) const;
// Return the last sequence number.
uint64_t LastSequence() const { return last_sequence_; }
// Set the last sequence number to s.
void SetLastSequence(uint64_t s) {
assert(s >= last_sequence_);
last_sequence_ = s;
}
// Mark the specified file number as used.
void MarkFileNumberUsed(uint64_t number);
// Return the current log file number.
uint64_t LogNumber() const { return log_number_; }
// Return the log file number for the log file that is currently
// being compacted, or zero if there is no such log file.
uint64_t PrevLogNumber() const { return prev_log_number_; }
// Pick level and inputs for a new compaction.
// Returns nullptr if there is no compaction to be done.
// Otherwise returns a pointer to a heap-allocated object that
// describes the compaction. Caller should delete the result.
Compaction* PickCompaction();
// Return a compaction object for compacting the range [begin,end] in
// the specified level. Returns nullptr if there is nothing in that
// level that overlaps the specified range. Caller should delete
// the result.
Compaction* CompactRange(int level, const InternalKey* begin,
const InternalKey* end);
// Return the maximum overlapping data (in bytes) at next level for any
// file at a level >= 1.
int64_t MaxNextLevelOverlappingBytes();
// Create an iterator that reads over the compaction inputs for "*c".
// The caller should delete the iterator when no longer needed.
Iterator* MakeInputIterator(Compaction* c);
// Returns true iff some level needs a compaction.
bool NeedsCompaction() const {
Version* v = current_;
return (v->compaction_score_ >= 1) || (v->file_to_compact_ != nullptr);
}
// Add all files listed in any live version to *live.
// May also mutate some internal state.
void AddLiveFiles(std::set<uint64_t>* live);
// Return the approximate offset in the database of the data for
// "key" as of version "v".
uint64_t ApproximateOffsetOf(Version* v, const InternalKey& key);
// Return a human-readable short (single-line) summary of the number
// of files per level. Uses *scratch as backing store.
struct LevelSummaryStorage {
char buffer[100];
};
const char* LevelSummary(LevelSummaryStorage* scratch) const;
private:
class Builder;
friend class Compaction;
friend class Version;
bool ReuseManifest(const std::string& dscname, const std::string& dscbase);
void Finalize(Version* v);
void GetRange(const std::vector<FileMetaData*>& inputs, InternalKey* smallest,
InternalKey* largest);
void GetRange2(const std::vector<FileMetaData*>& inputs1,
const std::vector<FileMetaData*>& inputs2,
InternalKey* smallest, InternalKey* largest);
void SetupOtherInputs(Compaction* c);
// Save current contents to *log
Status WriteSnapshot(log::Writer* log);
void AppendVersion(Version* v);
Env* const env_;
const std::string dbname_;
const Options* const options_;
TableCache* const table_cache_;
const InternalKeyComparator icmp_;
uint64_t next_file_number_;
uint64_t manifest_file_number_;
uint64_t last_sequence_;
uint64_t log_number_;
uint64_t prev_log_number_; // 0 or backing store for memtable being compacted
// Opened lazily
WritableFile* descriptor_file_;
log::Writer* descriptor_log_;
Version dummy_versions_; // Head of circular doubly-linked list of versions.
Version* current_; // == dummy_versions_.prev_
// Per-level key at which the next compaction at that level should start.
// Either an empty string, or a valid InternalKey.
std::string compact_pointer_[config::kNumLevels];
};
定义很长，先放着，继续看实现的部分（顺序经过重排）：
void Version::Ref() { ++refs_; }
void Version::Unref() {
assert(this != &vset_->dummy_versions_);
assert(refs_ >= 1);
--refs_;
if (refs_ == 0) {
delete this;
}
}
Version::~Version() {
assert(refs_ == 0);
// Remove from linked list
prev_->next_ = next_;
next_->prev_ = prev_;
// Drop references to files
for (int level = 0; level < config::kNumLevels; level++) {
for (size_t i = 0; i < files_[level].size(); i++) {
FileMetaData* f = files_[level][i];
assert(f->refs > 0);
f->refs--;
if (f->refs <= 0) {
delete f;
}
}
}
}
VersionSet::VersionSet(const std::string& dbname, const Options* options,
TableCache* table_cache,
const InternalKeyComparator* cmp)
: env_(options->env),
dbname_(dbname),
options_(options),
table_cache_(table_cache),
icmp_(*cmp),
next_file_number_(2),
manifest_file_number_(0), // Filled by Recover()
last_sequence_(0),
log_number_(0),
prev_log_number_(0),
descriptor_file_(nullptr),
descriptor_log_(nullptr),
dummy_versions_(this),
current_(nullptr) {
AppendVersion(new Version(this));
}
VersionSet::~VersionSet() {
current_->Unref();
assert(dummy_versions_.next_ == &dummy_versions_); // List must be empty
delete descriptor_log_;
delete descriptor_file_;
}
void VersionSet::AppendVersion(Version* v) {
// Make "v" current
assert(v->refs_ == 0);
assert(v != current_);
if (current_ != nullptr) {
current_->Unref();
}
current_ = v;
v->Ref();
// Append to linked list
v->prev_ = dummy_versions_.prev_;
v->next_ = &dummy_versions_;
v->prev_->next_ = v;
v->next_->prev_ = v;
}
VersionSet 构造函数的参数包括数据库的 name 和 options，缓存 table_cache 以及内部比较器 cmp。大部分成员变量都初始化为 0 或 nullptr，值得注意的是 next_file_number_=2，还有 dummy_versions_(this)。dummpy_versions_ 注释中说明了是版本双向链表的头部，不会有其他实际功能。构造函数中会执行 AppendVersion 增加一个新版本，也就是在双向链表的尾部插入版本 v，并且将 current_ 指向这个最新的版本；而析构函数中会要求当 current_ 降低引用计数、完成可能的析构后，dummy_versions_ 所指向的双向链表为空。继续看：
Status VersionSet::LogAndApply(VersionEdit* edit, port::Mutex* mu) {
if (edit->has_log_number_) {
assert(edit->log_number_ >= log_number_);
assert(edit->log_number_ < next_file_number_);
} else {
edit->SetLogNumber(log_number_);
}
if (!edit->has_prev_log_number_) {
edit->SetPrevLogNumber(prev_log_number_);
}
edit->SetNextFile(next_file_number_);
edit->SetLastSequence(last_sequence_);
Version* v = new Version(this);
{
Builder builder(this, current_);
builder.Apply(edit);
builder.SaveTo(v);
}
Finalize(v);
// Initialize new descriptor log file if necessary by creating
// a temporary file that contains a snapshot of the current version.
std::string new_manifest_file;
Status s;
if (descriptor_log_ == nullptr) {
// No reason to unlock *mu here since we only hit this path in the
// first call to LogAndApply (when opening the database).
assert(descriptor_file_ == nullptr);
new_manifest_file = DescriptorFileName(dbname_, manifest_file_number_);
edit->SetNextFile(next_file_number_);
s = env_->NewWritableFile(new_manifest_file, &descriptor_file_);
if (s.ok()) {
descriptor_log_ = new log::Writer(descriptor_file_);
s = WriteSnapshot(descriptor_log_);
}
}
// Unlock during expensive MANIFEST log write
{
mu->Unlock();
// Write new record to MANIFEST log
if (s.ok()) {
std::string record;
edit->EncodeTo(&record);
s = descriptor_log_->AddRecord(record);
if (s.ok()) {
s = descriptor_file_->Sync();
}
if (!s.ok()) {
Log(options_->info_log, "MANIFEST write: %s\n", s.ToString().c_str());
}
}
// If we just created a new descriptor file, install it by writing a
// new CURRENT file that points to it.
if (s.ok() && !new_manifest_file.empty()) {
s = SetCurrentFile(env_, dbname_, manifest_file_number_);
}
mu->Lock();
}
// Install the new version
if (s.ok()) {
AppendVersion(v);
log_number_ = edit->log_number_;
prev_log_number_ = edit->prev_log_number_;
} else {
delete v;
if (!new_manifest_file.empty()) {
delete descriptor_log_;
delete descriptor_file_;
descriptor_log_ = nullptr;
descriptor_file_ = nullptr;
env_->DeleteFile(new_manifest_file);
}
}
return s;
}
void VersionSet::Finalize(Version* v) {
// Precomputed best level for next compaction
int best_level = -1;
double best_score = -1;
for (int level = 0; level < config::kNumLevels - 1; level++) {
double score;
if (level == 0) {
// We treat level-0 specially by bounding the number of files
// instead of number of bytes for two reasons:
//
// (1) With larger write-buffer sizes, it is nice not to do too
// many level-0 compactions.
//
// (2) The files in level-0 are merged on every read and
// therefore we wish to avoid too many files when the individual
// file size is small (perhaps because of a small write-buffer
// setting, or very high compression ratios, or lots of
// overwrites/deletions).
score = v->files_[level].size() /
static_cast<double>(config::kL0_CompactionTrigger);
} else {
// Compute the ratio of current size to size limit.
const uint64_t level_bytes = TotalFileSize(v->files_[level]);
score =
static_cast<double>(level_bytes) / MaxBytesForLevel(options_, level);
}
if (score > best_score) {
best_level = level;
best_score = score;
}
}
v->compaction_level_ = best_level;
v->compaction_score_ = best_score;
}
核心接口 LogAndApply 会根据当前版本 current_ 和修订部分 edit，合成一个新版本 v，而后将修订的记录写入 Manifest 文件中，最后将新版本 AppendVersion 到版本集中作为新的 current_，这样就完成了一个新版本的构建。而 Recover 则对应从 Manifest 中恢复版本的过程。由于 Manifest 记录了所有的版本变更信息，使用 VersionSet::Builder 逐个 Apply 就可以获得存储的最新版本：
Status VersionSet::Recover(bool* save_manifest) {
struct LogReporter : public log::Reader::Reporter {
Status* status;
void Corruption(size_t bytes, const Status& s) override {
if (this->status->ok()) *this->status = s;
}
};
// Read "CURRENT" file, which contains a pointer to the current manifest file
std::string current;
Status s = ReadFileToString(env_, CurrentFileName(dbname_), ¤t);
if (!s.ok()) {
return s;
}
if (current.empty() || current[current.size() - 1] != '\n') {
return Status::Corruption("CURRENT file does not end with newline");
}
current.resize(current.size() - 1);
std::string dscname = dbname_ + "/" + current;
SequentialFile* file;
s = env_->NewSequentialFile(dscname, &file);
if (!s.ok()) {
if (s.IsNotFound()) {
return Status::Corruption("CURRENT points to a non-existent file",
s.ToString());
}
return s;
}
bool have_log_number = false;
bool have_prev_log_number = false;
bool have_next_file = false;
bool have_last_sequence = false;
uint64_t next_file = 0;
uint64_t last_sequence = 0;
uint64_t log_number = 0;
uint64_t prev_log_number = 0;
Builder builder(this, current_);
{
LogReporter reporter;
reporter.status = &s;
log::Reader reader(file, &reporter, true /*checksum*/,
0 /*initial_offset*/);
Slice record;
std::string scratch;
while (reader.ReadRecord(&record, &scratch) && s.ok()) {
VersionEdit edit;
s = edit.DecodeFrom(record);
if (s.ok()) {
if (edit.has_comparator_ &&
edit.comparator_ != icmp_.user_comparator()->Name()) {
s = Status::InvalidArgument(
edit.comparator_ + " does not match existing comparator ",
icmp_.user_comparator()->Name());
}
}
if (s.ok()) {
builder.Apply(&edit);
}
if (edit.has_log_number_) {
log_number = edit.log_number_;
have_log_number = true;
}
if (edit.has_prev_log_number_) {
prev_log_number = edit.prev_log_number_;
have_prev_log_number = true;
}
if (edit.has_next_file_number_) {
next_file = edit.next_file_number_;
have_next_file = true;
}
if (edit.has_last_sequence_) {
last_sequence = edit.last_sequence_;
have_last_sequence = true;
}
}
}
delete file;
file = nullptr;
if (s.ok()) {
if (!have_next_file) {
s = Status::Corruption("no meta-nextfile entry in descriptor");
} else if (!have_log_number) {
s = Status::Corruption("no meta-lognumber entry in descriptor");
} else if (!have_last_sequence) {
s = Status::Corruption("no last-sequence-number entry in descriptor");
}
if (!have_prev_log_number) {
prev_log_number = 0;
}
MarkFileNumberUsed(prev_log_number);
MarkFileNumberUsed(log_number);
}
if (s.ok()) {
Version* v = new Version(this);
builder.SaveTo(v);
// Install recovered version
Finalize(v);
AppendVersion(v);
manifest_file_number_ = next_file;
next_file_number_ = next_file + 1;
last_sequence_ = last_sequence;
log_number_ = log_number;
prev_log_number_ = prev_log_number;
// See if we can reuse the existing MANIFEST file.
if (ReuseManifest(dscname, current)) {
// No need to save new manifest
} else {
*save_manifest = true;
}
}
return s;
}
bool VersionSet::ReuseManifest(const std::string& dscname,
const std::string& dscbase) {
if (!options_->reuse_logs) {
return false;
}
FileType manifest_type;
uint64_t manifest_number;
uint64_t manifest_size;
if (!ParseFileName(dscbase, &manifest_number, &manifest_type) ||
manifest_type != kDescriptorFile ||
!env_->GetFileSize(dscname, &manifest_size).ok() ||
// Make new compacted MANIFEST if old one is too big
manifest_size >= TargetFileSize(options_)) {
return false;
}
assert(descriptor_file_ == nullptr);
assert(descriptor_log_ == nullptr);
Status r = env_->NewAppendableFile(dscname, &descriptor_file_);
if (!r.ok()) {
Log(options_->info_log, "Reuse MANIFEST: %s\n", r.ToString().c_str());
assert(descriptor_file_ == nullptr);
return false;
}
Log(options_->info_log, "Reusing MANIFEST %s\n", dscname.c_str());
descriptor_log_ = new log::Writer(descriptor_file_, manifest_size);
manifest_file_number_ = manifest_number;
return true;
}
void VersionSet::MarkFileNumberUsed(uint64_t number) {
if (next_file_number_ <= number) {
next_file_number_ = number + 1;
}
}
本篇分析了版本管理相关的代码，包括 VersionEdit、Version 和 VersionSet 的实现。VersionSet 还有一大部分代码是与 Compaction 相关的，将会在下篇中继续分析。